[derringer]

Identifying power lines and people isn't necessarily as hard as you say. With a simple image, it's true machines can't match human perception. But when you take into account infrared for viewing human profiles, magnetic sensors for checking for power lines and the fact that the bots are mobile and can investigate and base their analysis on 3d space. The computational bang for your buck is also continuing to improve and sensors are continuing to get smaller. These systems aren't possible yet, but they may be in the relatively near future.